1. Field of the Invention
The present invention relates to a dismantling method and a recycling method for a magnetic field generator, and more specifically to a dismantling method and a recycling method for a magnetic field generator comprising neodymium magnets and used in a large MRI apparatus capable of scanning a human body.
2. Description of the Related Art
Size of the neodymium magnet, which is a sintered body, is limited. Usually therefore, in this kind of large magnetic field generator, a plurality of neodymium magnets are fixed to a plate yoke by an adhesive.
A method for dismantling such a magnetic field generator can be as follows: Specifically, screws connecting the plate yokes to column yokes are loosened, then the magnetic field generator is suspended by a crane, and disassembled. Each of the plate yokes is then bathed in a drum filled with solvent for melting the adhesive, and the neodymium magnets fixed to the plate yokes are taken out.
However, the neodymium magnets, as magnetized, have a very strong magnetism. Therefore, according to the above-mentioned method, it is dangerous because the neodymium magnets will be thrown out of the drum by strong repelling force among the neodymium magnets once the adhesive loses its adhering strength. Another potential danger is that a worker""s hand can be caught between two neodymium magnets for example, by strong magnetic attraction, when one of the neodymium magnets is pulled off from another.
Further, the neodymium magnet is a sintered body which is fragile. Thus, once the adhesive loses its adhering strength, if the magnet is thrown by a magnetic repelling force acting among the neodymium magnets and hits something, the magnet will be chipped.
Further, if the neodymium magnet hits another neodymium magnet or the plate yoke, a spark will be generated, which may cause fire or explosion accident.
There may be an idea that the neodymium magnets are demagnetized by applying demagnetizing magnetic field at the time of dismantling the magnetic field generator. However, the magnetic field generator for MRI apparatus is large, and therefore demagnetizing operation, in which a strong and large magnetic field must be generated, is difficult. A large demagnetizing equipment is not realistic due to a high cost.
The Japanese Patent Publication (of examined Application for opposition) No. 3-20045 discloses a method, in which each of permanent magnets is first magnetized, then measured for its magnetic characteristic, then heated to demagnetize, and at a time of assembly, each of the magnets is re-magnetized in accordance with its magnetic characteristic for assembly into a magnet body. However, this method is for appropriately disposing the permanent magnets, and nothing has been disclosed for a method for dismantling or recycling the large magnetic field generator.
It is therefore a primary object of the present invention to provide a method for dismantling the magnetic field generator and a method of recycling the same, in which the dismantling and recycling can be made safely and at a low cost.
According to an aspect of the present invention, there is provided a dismantling method for a magnetic field generator comprising a plate yoke, and a permanent magnet provided on the plate yoke and including a plurality of neodymium magnets bonded together by an adhesive, wherein the magnetic field generator is heated at a temperature of 200xc2x0 C.xcx9c1000xc2x0 C.
If the heating temperature of the magnetic field generator is lower than 200xc2x0 C., the neodymium magnets cannot be sufficiently demagnetized and cannot be taken out safely. Further, the adhesive has reversibility up to 200xc2x0 C., and therefore if the heating temperature is lower than 200xc2x0 C., then adhering strength will come back when the adhesive is cooled thereafter. On the other hand, if the heating temperature exceeds 1000xc2x0 C., structure of the neodymium magnets is altered and its magnetic characteristic is deteriorated, making difficult to reuse the collected neodymium magnets. Therefore, by heating the magnetic field generator at a temperature of 200xc2x0 C.xcx9c1000xc2x0 C., the neodymium magnets are sufficiently demagnetized and the adhering strength of the adhesive is reduced. As a result, the neodymium magnets can be taken out safely, and the magnetic field generator can be dismantled safely. Further, the simple heating operation makes possible to reduce cost.
Preferably, the magnetic field generator further comprises a column yoke connected to the plate yoke. The as-magnetized neodymium magnets are very dangerous, but by heating the magnetic field generator in a state in which the column yoke is connected to the plate yoke, handling of the magnetic field generator at the time of dismantling becomes easier.
Further, preferably, the heating temperature of the magnetic field generator is 200xc2x0 C.xcx9c400xc2x0 C. Due to a characteristic of the neodymium magnets, aging becomes necessary if the heating temperature exceeds 400xc2x0 C. Thus, by heating at a temperature not higher than 400xc2x0 C., it becomes possible to reuse the neodymium magnets by only re-magnetizing the heated neodymium magnets without performing the aging operation.
Preferably, the heating temperature of the magnetic field generator is 200xc2x0 C.xcx9c350xc2x0 C., and at least one of the neodymium magnets is removed by first demagnetizing the neodymium magnet and then removing the adhesive. By heating at a temperature of 200xc2x0 C.xcx9c350xc2x0 C. as described above, the neodymium magnets are demagnetized, the adhesive is degraded, and the adhering strength of the adhesive is reduced. Thus, by only removing the degraded adhesive, the neodymium magnets can be taken out safely. Further, since surfaces of the neodymium magnets are not severely deteriorated, the neodymium magnets can be reused easily.
Further, preferably, the heating temperature of the magnetic field generator is 350xc2x0 C.xcx9c1000xc2x0 C., and at least one of the neodymium magnets is removed by carbonizing the adhesive. By heating the magnetic field generator at a temperature not lower than 350xc2x0 C., the adhesive is carbonized to lose the adhering strength and the neodymium magnets lose the magnetism almost completely. Therefore, the removal and handling of the neodymium magnets becomes easy.
Still further, preferably, the adhesive is an acrylic adhesive. The acrylic adhesive has a high adhering strength under a room temperature, yet the adhering strength is reduced due to thermal degradation or carbonization if heated at a temperature exceeding 200xc2x0 C. Therefore, the removal of the neodymium magnets becomes easy if the acrylic adhesive is used as the adhesive.
Preferably, the neodymium magnets are three-element neodymium magnets having a Rxe2x80x94Fexe2x80x94B (R is rare earth element) composition. The three-element neodymium magnet, as compared to a four-element neodymium magnet which includes Co, has a lower magnetic flux density and coercive force at a high temperature range, starts losing the magnetic force at a lower temperature, and has a greater demagnetizing rate. Therefore, if the three-element neodymium magnets are used, desired demagnetization can be achieved easily to the neodymium magnets, making the magnetic field generator suitable for recycling by heating.
Further, preferably, magnetic poles of the neodymium magnets are oriented in the same direction. If the magnetic poles of the neodymium magnets are oriented in the same direction, the magnetism generated becomes more intense. However, according to the present invention, the neodymium magnets can be collected safely even under such a situation.
According to another aspect of the present invention, there is provided a recycling method for a magnetic field generator comprising a plate yoke, and a permanent magnet provided on the plate yoke and including a plurality of neodymium magnets bonded together by an adhesive, wherein the magnetic field generator is heated to 200xc2x0 C.xcx9c1000xc2x0 C., then at least one of the neodymium magnets is removed, and a surface of the removed neodymium magnet is polished for reusing the neodymium magnet.
By polishing the surfaces of the removed neodymium magnets, the surfaces are cleaned and adhesion necessary for re-bonding the neodymium magnets are recovered, enabling the recycling of the neodymium magnets.
According to still another aspect of the present invention, there is provided a recycling method for a magnetic field generator comprising a plate yoke, and a permanent magnet provided on the plate yoke and including a plurality of neodymium magnets bonded together by an adhesive, wherein the magnetic field generator is heated to 200xc2x0 C.xcx9c1000xc2x0 C., then at least one of the neodymium magnets is removed, and the removed neodymium magnet is re-aged for reusing.
By re-aging the removed neodymium magnets, the characteristic of the neodymium magnets can be reliably reestablished, enabling smooth recycling of the neodymium magnets.